二项式求问题，为什么出来的数字超级超级长?

#include<stdio.h>

#include<math.h>

int main()

{

   double f1(double a[], double x, int n);

    double a[6];

    f1(a, 1.0, 5);

printf("%lf", f1(a, 1.0, 5));

return 0;

}

double f1(double a[], double x, int n)

{

double total = a[0];  

int i;

for (i = 1;i < n;i++)

{    

a[i] = (double)i;

total += a[i] * pow(x, i);

}

return total;

}